This nonprovisional application claims priority under 35 U.S.C. xc2xa7 119(a) on Patent Application No. 0130152.2 filed in Great Britain on Dec. 18, 2001, which is herein incorporated by reference.
This invention relates to electric motors and in particular, to miniature electric motors having a permanent magnet stator.
PMDC motors are very common and have many uses and applications. Some applications require motors with a relatively high power with small physical size while some applications require the motor to have low cogging.
Cogging can be reduced by shaping the rotor poles and/or the stator poles. As the stator is a permanent magnet, the shaping of the stator poles means shaping the magnets or the magnetic field which can be shaped during the charging of the magnets. This is usually only possible with ring magnets but ceramic ring magnets are very expensive and rubber ring magnets are not as strong.
When using ceramic magnets, it is usual to use two arcuate ceramic magnets which are circumferentially located with respect to the can-like housing by stops which engage one longitudinal side of each magnet and one or more springs located between and bearing against the other longitudinal edge of the magnets, pressing the magnets into contact with the stops.
A similar arrangement is used with four arcuate magnets and two steps of stops.
Once installed in the housing, the magnets are charged. However, the gap between the ceramic magnets represents a gap in the magnetic field of the stator and any shaping of the magnetic field must take this gap into consideration.
The rubber ring magnet, on the other hand, requires no springs and no stops pressed into the wall of the housing. The magnet is held in place by the magnet itself which expands when it is charged resulting in a housing without side openings. However, there is a need for a housing without side openings which supports a more powerful magnet such as a ceramic magnet.
Accordingly, the present invention provides a permanent magnet motor having a cylindrical housing containing a permanent magnet stator and a wound rotor, wherein the permanent magnet stator comprises at least two arcuate ceramic magnets circumferentially spaced by rubber magnets. The present invention also provides a method of making such a motor.